Study on fluid inclusions and stable isotopics of W–Mo ore deposits in the Ningshan–Zhen’an area, South Qinling, China

The study of the fluid inclusions of W–Mo deposits in the mineralization area of Ningshan–Zhen’an , Shaanxi Province, China shows that the gas–liquid two-phase inclusions are mainly present in W–Mo deposits, and the ore-forming fluid can be divided into four types: high-temperature type, middle–high-temperature type, middle-temperature type and low-temperature type. The formation depths of the W–Mo mineralization range from 4.2 to 8.4 km. The boiling and mixing of fluid may have been important mechanisms for the formation of W–Mo mineralization. The skarn-type mineralization is dominated by magmatic water, the quartz-vein-type mineralization includes both magmatic water and meteoric water, and the meteoric water is more involved in the quartz-fluorite-vein-type, beryl-quartz-vein-type and pegmatite-type mineralization. Magma is the main source of sulfur; that is, magma is the main source of mineralization. Combined with the metallogenic setting and geological characteristics of typical ore deposits, in the process of structural system transformation in South Qinling, the ore-forming magma fluid in the Late Indosinian–Yanshanian period was uplifted and emplaced along the NW–WNW direction and NE–NNE direction, and eventually, NW–WNW fault-controlled skarn-type W–Mo mineralization and quartz-vein-type W–Mo deposits accompanied by greisenization, albitization and potash feldspathization formed.

been found Moreover, the geological characteristics of the W-Mo deposits [13][14][15] , the geochemical characteristics of the deposits [16][17][18][19] , the ore-controlling structures [20][21][22] , the metallogenic epoch 19,[23][24][25] , and the genesis of the deposits 13,16,17,19 have been studied, providing a strong theoretical basis for subsequent exploration work in the mine area.However, the sources of ore-forming fluids and metals, as well as the metal enrichment processes in the mineralization area of Ningshan-Zhen'an, are still poorly understood.In this contribution, we present field observations, petrographic, microthermometric and Raman analyses of fluid inclusions combined with new stable H, O and S isotope studies of the Ningshan-Zhen'an W-Mo deposits to determine the origin of the oreforming materials and evolutionary processes of the hydrothermal fluid system.These results can also provide significant insight into the mechanisms of large-scale ore precipitation, which could improve our understanding of ore genesis in the mineralization area of Ningshan-Zhen'an and other W-Mo deposits throughout the southern Qinling or Qinling orogenic belt.

Regional geology
The mineralization area in Ningshan-Zhen'an is located in the tectonic area where the nearly E-W tectonic belt of the Indosinian collision orogenic belt and the nearly N-S tectonic belt of the Yanshanian intracontinental orogenic belt intersect in the South Qinling orogenic belt (Fig. 1b).The outcropping strata are mainly fine clastic-carbonate rocks of neritic shelf facies in the Devonian to upper Paleozoic strata, and there are local clastic

S-isotope analyses
In this work, 9 ore samples from the Qipangou, Yueheping, Guilingou, Daxigou, and Yanggou-Di' ergou mine areas were first crushed to 40-80 mesh, after which molybdenite and pyrite with purities greater than 95% were selected under a binocular microscope and then ground to 200 mesh for sulfur isotope analysis.Sulfur isotope testing was performed at the Beijing Research Institute of Uranium Geology.The use of instruments and the detailed test process are detailed in the references 34,35 .

Petrographic characteristics of fluid inclusions
The fluid inclusions in the minerals, which are mainly ellipsoids, are well developed (Supplementary Table 2), and their sizes were 5-20 μm.According to Lu et al. 36 , the fluid inclusions in the Ningshan-Zhen'an area can be classified into vapor-liquid phases (type I), pure-liquid phases (type II) and pure-vapor phases (type III) (Supplementary Fig. 3).
Vapor-liquid two-phase (type I): This type is composed of a vapor phase and aqueous solution and mostly develops in fluorite and quartz mineral crystals.This type of inclusion can be further divided into a liquid-rich phase (type I-a, with more than 70% liquid phase) and a vapor-rich phase (type I-b).Type I-a comprises elliptically shaped fluid inclusions up to 21 μm in diameter, primarily distributed in a linear and isolated pattern within quartz crystals, with the volume fraction of the vapor phase from 90 to 92%(Supplementary Fig. 3a-c).The vapor-liquid ratio of type I-b inclusions is high (more than 40%), and they have relatively low abundances www.nature.com/scientificreports/ of quartz, fluorite and scheelite.The color of the inclusions are generally vivid, and the inclusions are generally large, with particle sizes of 10-20 μm and elliptical shapes (Supplementary Fig. 3d and e).Pure-liquid phase (type II): Single-phase inclusions are distributed in quartz and fluorite, mainly in scattered or isolated form, which are associated with the above types of inclusions, but the number of inclusions is small and elliptical.Type II inclusions are usually colorless and transparent (Supplementary Fig. 3d).
Pure-vapor phase (type III): Type III inclusions are generally grayish black and difficult to observe (Supplementary Fig. 3b and c).In addition to the above types, CO 2 -bearing and subcrystalline polyphase inclusions are rare (Supplementary Fig. 3f.).

Characteristics of the homogenization temperature and salinity of fluid inclusions
The freezing-point temperature and homogenization temperature of the inclusions were obtained by the freezing method and homogenization method, respectively.
Based on the statistical results of the homogenization temperature in the mineralization area and the research results of Liu 13 on fluid inclusions in the Dongyang mine area, the ore-forming fluids of Mo and W mineralization in the mineralization area can be classified into four types (Supplementary Table 2 and Supplementary Fig.

Density characteristics of fluid inclusions
Based on the density formula of vapor-liquid two-phase inclusions 33,35,37,38 , the statistical results of the fluid inclusion density in the mineralization area are shown in Supplementary Table 2.The densities of the ore-forming fluids of quartz-vein-type, pegmatite-type, quartz-fluorite-vein-type, etc., in other mine areas are similar, except for the densities of the quartz-vein-type and skarn-type ore-forming fluids in the Qipangou mine area, which are relatively small.

Capture pressure and depth
The capture pressure of the fluid inclusions in the quartz veins was calculated, and the capture depth was estimated based on the empirical formula 39,40 and a Microsoft Excel spreadsheet based on the PVTX (pressure-volume-temperature-composition) properties of H 2 O-NaCl (Steele-MacInnis et al.) 41 of fluid pressure.According to the estimated fluid-capture pressure-metallogenic depths (Supplementary Table 2), the formation depths of the skarn-type and quartz-vein-type W mineralization in the Qipangou mine area are the greatest, ranging from 8.09 to 8.44 km.The depths of the Mo-W mineralization of the quartz-vein-type in other mine areas are 6.08-7.77km, which are slightly greater than that of the Qitangou mine area.The depths of the pegmatite-type mineralization are 5.09-5.82km, which is greater than that of the quartz-vein-type mineralization.The quartzfluorite-vein-type mineralization and beryl-quartz-vein-type mineralization are located in the shallowest parts of the mineralization area at depths of approximately 4.23-5.48km.The vertical zonation at the different mineralization depths is similar to the distribution characteristics of the homogenization temperature and salinity above.

Laser Raman analysis of inclusions
The results of the laser-Raman microprobe analysis show that the vapor-phase compositions of the quartz inclusions in the ore-bearing quartz veins are mainly CO 2 and CH 4 , while the contents of N 2 , SO 2 and H 2 O are not significant; H 2 O is the main component in the liquid phase (Supplementary Fig. 5).In the skarn-type mineralization, there is a single vapor component in the quartz inclusions, mainly CO 2 , and the liquid component is mainly H 2 O.The vapor components of the pegmatite-type mineralization are mainly CO 2 and SO 2 , which is consistent with Liu's research 13 on Dongyang W deposits.

H-O isotopes
The H-O isotope analysis results for the quartz inclusions are shown in Supplementary Table 3.
The test results of the quartz in the different W-Mo mineralization types in the mineralization area show that the δD values range from − 64.9 to − 80.1‰ and that the mean value is − 74.4‰, which is within the range of normal magmatic water.The δ 18 O values of the quartz in the different W-Mo mineralization types in the mineralization area range from 7.3 to 13.8‰, and the mean value is 11.4‰, which is consistent with the δ 18 O values of crust-remelting granites (10.0-12.0‰) 42.According to the quartz-water equilibrium fractionation Equation 43 , the final δ 18 O H2O values are − 1.71-6.42‰,and the mean value is 2.67‰.Overall, the δD values vary little due to the small effect of water-rock interactions on the δD values, and the H isotopes of the different ore-forming types are similar; however, from skarn-type mineralization to quartz-vein-type mineralization, followed by beryl-quartz-vein-type, quartz-fluorite-vein-type and pegmatite-type mineralization, the O isotopes decrease in turn.On the H-O isotope diagrams, all the samples in the Qipangou mine area are in the magmatic water field, and the skarn-type mineralization in the Dongyang mine area plots within the initial magmatic-water range of the W-Sn series.The other samples are all located between the magmatic-water range and the meteoric water line.The quartz-fluorite-vein-type, beryl-quartz-vein-type and pegmatite-type mineralization continue to drift to the meteoric water line, indicating that magmatic water was the main component of the ore-forming fluid in the skarn-type mineralization stage.During the hydrothermal-vein mineralization stage, the hydrothermal fluid gradually mixed with meteoric water.In the late stages of quartz-fluorite-vein-type, beryl-quartz-vein-type and pegmatite-type mineralization, meteoric water was more involved in mineralization (Fig. 2).www.nature.com/scientificreports/

S isotopes
The results of the analysis of the S isotopes are shown in Supplementary Table 4.
The test results showed the following.The δ 34 S values of the ore-bearing quartz veins are 3.6-10.2‰,with a mean value of 7.3‰.The δ 34 S value of the skarn-type mineralization is 6.1‰, the δ 34 S value of the pegmatite-type mineralization is 4‰, and the δ 34 S value of the granite-porphyry-type Mo deposit of Daxigou is 0.1‰.All the samples demonstrate positive values and have magmatic sulfur features (Fig. 3).The δ 34 S of the granite-porphyry Mo deposit of Daxigou is nearly 0, which indicates that it is a deep-source sulfur deposit.The δ 34 S values of quartz-vein-type, skarn-type and pegmatite-type mineralization range from 3.6 to 10.2‰, which are essentially within the δ 34 S range of granitic magma (5-15‰; Fig. 3) 46 .The above data indicate that sulfur may mainly arise from deep magmas and may be contaminated by surrounding rocks, which indicates that the S isotopes are gradually homogenized through crust-mantle interactions during the metallogenic process; the ore-forming fluid may have arisen from a unified fluid system of reservoir-crust-mantle interaction processes, and the ore-forming material in the ore concentration area may have mainly arisen from granite (acidic igneous rock).

Nature and evolution of the fluids
Based on the results of the homogenization temperature and salinity of the fluid inclusions, the fluid salinity of the quartz-vein-type inclusions in the Dongyang mine area is greater than that of the pegmatite-type and quartz-fluorite-vein-type inclusions.In the Qipangou mine area, the fluid salinities of the skarn-type and quartz-vein-type are similar, and the span is large.In the Hetaoping mine area, the fluid salinity of the quartzvein-type is greater than that of the beryl quartz-vein-type.In the Yanggou-Di' ergou mine area, the fluid salinity of the quartz-vein-type is generally greater than that of the pegmatite type and the quartz fluorite type.The fluid salinity in the mineralization area generally shows the same distribution characteristics as the homogenization temperature; namely, the quartz-vein-type and skarn-type ore-forming fluids generally exhibit intermediate-high temperatures and moderate-high salinities.The pegmatite type, quartz-fluorite-vein type and beryl-quartz-vein type show the characteristics of middle-low-temperature and middle-low-salinity fluids.
The quartz, fluorite and scheelite crystals of different types of mineralization in the mineralization area feature the coexistence of various types of inclusions with similar homogenization temperatures, and fluid boiling may have occurred 36 ; this is also one of the main reasons for the mineralization of W, Mo and other metals 38 .The skarn-type Mo-W deposits and quartz-vein-type Mo-W deposits are metallogenic fluids with intermediate-high temperatures and high salinities and may boil at 355-380 °C, resulting in the precipitation of early W-Mo  48,49 .The compositions of quartz-bearing fluid inclusions with different mineralization types or stages in the mineralization area were analyzed and compared.There was more CH 4 and less CO 2 in the quartz-fluid inclusions of the quartz-vein-type W mineralization in the Qipangou mine area (Supplementary Fig. 5c); the results show that the fluid was mainly reductive, which is favorable for the transport of W.There was little or no CH 4 and less CO 2 in the skarn-type mineralization, which may have been related to the number of samples detected (only one sample was detected in skarn-type mineralization).In the quartz-bearing fluid inclusions from the Dongyang, Hetaoping and Yanggou-Di' ergou mine areas, the CH 4 content obviously decreased, while the CO 2 content increased; moreover, there was little SO 2 and N 2 in some of these areas, indicating that the fluid reducibility decreased and slightly deviated from oxidation and that other source fluids may have been added.Pegmatite-type mineralization does not contain CH 4 , and in vapor-phase compositions, CO 2 and SO 2 are common; therefore, it is also a partially oxidized fluid.The results of the analysis of the redox properties of the fluid inclusions are consistent with those of the above studies on temperature, salinity and estimated depth, which indicate the metallogenic environment.The quartz-vein-type ore-forming fluid in the Qipangou mine area are medium-high-temperature and medium-high-salinity; the mineralization occurred in the reducing deep environment of the mineralization areas.The quartz-vein-type ore-forming fluid in the Dongyang, Hetaoping and Yanggou-Di' ergou mine areas are medium-temperature and medium-low-salinity, which formed in a reduction-partial-oxidation environment in the shallow part of the mineralization area.The pegmatite-type ore-forming fluid had low temperatures and salinities; it formed in the shallow oxidizing environment of the mineralization area.

Source of ore-forming fluid and mechanism of mineral precipitation
According to the research results of the fluid inclusions in the mineralization area, both magmatic water and meteoric water are present in the ore-forming fluid and may have boiled and mixed.The redox environments of the different mineralization types are also different.If there is only one stage of mineralization in the mineralization area, the early stage may have been a partially reduced environment, which gradually changed to a reducing-partially oxidizing environment with the upwelling evolution of the ore-forming fluid along the tectonic belt.Based on the geological characteristics of the surface outcrop and the existence of blind granite bodies in the depths of the mineralization area and the results of the S isotope measurements, it can be inferred that the ore-forming fluid in the mineralization area was mainly derived from hydrothermal magma in the deep areas.During the process of magmatic hydrothermal fluid ascending and migrating along the tectonic belt, the temperature gradually decreased, after which the magmatic hydrothermal fluid mixed with meteoric water in The δ 34 S values can be used to broadly identify three different S sources: (1) mantle-derived (or magmatic) S has δ 34 S values of 0 ± 3‰, (2) seawater-derived sulfur has δ 34 S values of ~ + 20‰, and (3) sedimentary S typically has highly variable but negative δ 34 S values due to the isotopic fractionation caused by bacteria and redox processes.Additionally, (4) mixed S can contain contributions from the three aforementioned sources and has variable δ 34 S values 47,50 .The S isotopic compositions of the typical deposits in the mineralization area are relatively concentrated, and the δ 34 S values are basically positive, indicating that the sulfur is mainly magmatic (Fig. 3).The results show that the positive δ 34 S values of the metal sulfides are mainly related to magmatic degassing or marine sulfates.Magmatic SO 2 degassing can lead to a significant loss of 34 S in the sulfides of rocks, and the δ 34 S value can change from 0 to − 8‰.Magmatic H 2 S degassing can lead to a relative enrichment of 34 S in the sulfides of rocks, and the δ 34 S value can change from 0 to 6‰ 43 .Therefore, the positive δ 34 S values of the W-Mo mineralization area in Ningshany-Zhen'an may be the result of magmatic degassing.SO 2 is generally detected in the vapor phase of fluid inclusions (Supplementary Fig. 5).SO 2 and H 2 S are two common S-containing gases in magmatic volatiles, while 34 S is more enriched in SO 2 than in H 2 S and melts; therefore, large amounts of H 2 S escaping from a magmatic system can lead to an increase in the δ 34 S values (> 0‰) of sulfides in melts or late ore-forming hydrothermal solutions 51 .Magmatic volatiles also tend to have higher N 2 contents 52 .The discovery of N 2 in the inclusions of the W-Mo mineralized quartz veins also suggests that magmatic fluids may be involved in mineralization.
At temperatures of 200-300 °C, NaWO 4 − and HWO 4 -are the main migration forms of W, while at 400-600 °C, NaHWO 4 is the main migration form 53,54 .The metallogenic temperatures of W and Mo in the W-Mo mineralization area in Ningshan-Zhen'an are mostly in the 300 °C range; therefore, NaWO 4 -and HWO 4 -may be the main forms of W in the ore-forming fluid in the mineralization area.
The vapor phase of fluid inclusions in mineralization areas generally contains CO 2 gas.Although a debate is underway as to whether the presence of CO 2 in fluids facilitates the migration and enrichment of W and Mo 55,56 , changes in the acidity and alkalinity of melt or ore-forming hydrothermal fluids and increases in metal concentrations are associated with CO 2 loss 57 .Other studies have shown that the loss of CO 2 can reduce the solubility of W in fluids, leading to precipitation and mineralization 58 .As mentioned above, typical fluid immiscibility or fluid boiling signs were observed in the quartz vein-mineralized fluid inclusions in the mineralization areas, and it was found that one of the causes of fluid boiling may be the mixing of different fluids 59 .Thus, the original equilibrium state of the fluid has been destroyed.Wood and Samson 60 and Redkin and Kostromin 61 have shown that changes in the temperature, Ph, Eh, etc., of ore-forming fluids have a significant effect on the stability of W-Mo complexes.When these physical and chemical factors change in the ore-forming fluid system, the complex of W and Mo easily decomposes, which leads to the enrichment, precipitation and mineralization of W and Mo.The temperature range of the ore-forming fluid in the mineralization area is large (between 100 and 500 °C).The main ore-forming temperatures in the mineralization area are between 200 and 300 °C and lower than 350 and 500 °C, except that the ore-forming temperature in the Qipangou mine area is generally higher than 300 °C, which is the general mineralization temperature of W and Mo 60 .This indicates that one of the main causes of W and Mo mineralization is fluid mixing and that the effect of cooling is less significant.
According to the metallogenic setting and typical geological characteristics of the W-Mo mineralization area in Ningshan-Zhen'an , during the transformation of the tectonic system in South Qinling, the tectonic-magmatic activities in the mineralization areas were intense; the extensional rock-controlling and ore-controlling fault systems of different scales developed widely and caused the late Indosinian-Yanshanian granitic magma to intrude into the lower part of the mineralization area, which created favorable conditions for the formation and migration of ore-forming fluids.This provided a structural pathway for the mixing of meteoric water seeping down to a certain depth in the Earth's crust and ore-forming hydrothermal fluid.In the ore-forming hydrothermal system after mixing, due to abrupt changes in physical chemistry, the physical chemistry system of the fluid balance was disrupted, and large amounts of ore-forming material began to accumulate and precipitate.

Conclusions
(1) The W-Mo mineralization in the mineralization area of Ningshan-Zhen'an is mainly skarn-and quartzvein-types and is controlled by NW-WNW and NE-NNE faults, joints, fissures, etc. W-Mo mineralization is usually located in the contact zone between the granitic body and carbonate rocks.water are present in quartz-vein-type mineralization, and in quartz-fluorite vein-type, beryl-quartz veintype and pegmatite-type mineralization, meteoric water is more heavily enriched in magma and is the main source of sulfur; that is, magma is the main source of mineralization.

Figure 1 .
Figure 1.Simplified maps of (a) the location of the Qinling orogenic belt within China; (b) tectonic divisions of the Qinling orogenic belt; and (c) granite distribution in the South Qinling belt (modified after Dong et al. 28 ).China basemap based on the China National Bureau of Surveying and Mapping Geographical Information.NCB, North China Block; S-NCB, southern margin of the North China Block; NQB, North Qinling belt; SQB, South Qinling belt; SCB, South China Block.The figure was generated using CorelDRAW X4 (https:// www.corel.com/ en/).

Figure 2 .
Figure 2. Diagram of δ 18 O H2O -δD isotopic-fluid properties in the mineralization area.The base map is based on Taylor 44 , and the initial magmatic water of the W-Sn series is based on Zhang 45 .

Figure 3 .
Figure 3. Distribution of the S-isotopic composition of W-Mo deposits in the mineralization area of Ningshan-Zhen'an.The base map is based on Hoefs 47 .

( 2 )
The ore-forming fluid related to the quartz-vein-type W deposits in the Qipangou mine area is middlehigh-temperature and middle-high-salinity fluid, which formed in the deep environment of the mineralization area, with partially reducing conditions.The quartz-vein-type ore-forming fluid in the Dongyang, Hetaoping and Yanggou-Di' ergou mine areas is intermediate-temperature and low-salinity; it formed in the shallowly reducing and partially oxidizing environment of the mineralization area.The pegmatite-type mineralized ore-forming fluid is low in temperature and salinity; it formed in a shallow, partially oxidizing environment.The depths of the Mo-W mineralization are 4.2 -8.4 km.(3)The skarn-type mineralization is mainly composed of magmatic water.Both magmatic water and meteoric